This invention relates to a method for producing dry fruit in the form of chip which is ready for eating as snack fruit and more particularly, to a method for producing dry fruit in the form of chip which provides a pleasant touch to the teeth and is ready for eating as snack fruit by drying fruit chips which have been previously adjusted to a suitable water-soluble sugar concentration at three different successive drying stages under different conditions particularly specified for these drying stages, respectively.
A number of methods for drying fruit chips have been proposed as well as those for drying vegetables to provide foods ready for eating. However, most of the prior art methods for drying fruit chips have been developed to provide fruit chips as instant foods which are adapted to be reconstituted with hot water when the chips are cooked or eaten after drying thereof. Thus, the obtained dry fruit chips are not those ready for eating as snack fruit and especially, such dry fruit chips fail to give such a pleasant touch to the teeth as called for snack fruit.
Dry fruit chips as snack fruit are not old in Japan and as a result, only a relatively small number of technologies have been developed up to date in Japan. Representative examples of the snack fruit technologies will be briefly described hereinbelow. Fruit is first removed its skin and core therefrom and then cut and trimmed to chips having a suitable shape and size. The fruit chips are immersed in one selected from various available syrup solutions and then subjected to a suitable drying treatment. The prior art drying methods include the oil-frying method, the freeze-drying method and the vacuum-drying method.
The oil-frying has the disadvantages that the oxidation of the oil deteriorates the flavor of the product and that the oil adversely affects the taste of the product (the taste becomes heavy).
The freeze-drying has the disadvantages that the drying requires a rather long time period and accordingly, is expensive and that the dry fruit chips obtained by the freeze-drying are suitable for reconstitution with hot water or the like because such fruit chips have a porous structure, but the porous structure lacks in pleasantness to the teeth as snack fruit ready for eating.
The vacuum-drying has the disadvantage that the obtained dry fruit chips have a reduced bulk rather than a porous structure and are too hard for snack fruit far from a pleasant touch to the teeth.
Therefore, the inventors have endeavoured to develop a method for producing dry fruit chips which are quite pleasant to the teeth and maintain their inherent bulk while eliminating the disadvantages inherent in the prior art methods for producing dry fruit chips ready for eating as snack fruit. And the inventors have found that in order to obtain dry fruit chips which are pleasant to the teeth, it is quite important to uniformly disperse a water-soluble sugar solution throughout fruit chips in a high concentration and dry the fruit chips while further increasing the concentration of the water-soluble sugar solution.
Through further strenuous research on fruit chip drying, the inventors have found that when fruit chips which have been subjected to a suitable pre-treatment are dried through three successive different drying stages, that is, freeze-drying, vacuum-drying and microwave-drying under vacuum, under different drying conditions specified for the three different drying stages, respectively, dry fruit chips pleasant to the teeth can be produced in a brief time period. It has been further found that the thus produced dry fruit chips retain their inherent flavor and taste and that the concentration of the water-soluble sugar in the fruit chips prior to the drying has an important influence upon the quality of the resultant dry fruit chips.
From the findings referred to above, the inventors have come to the drying principle for fruit chips in which fruit chips which have been adjusted to a specific water-soluble sugar concentration are first subjected to freeze-drying to evaporate a predetermined amount of water from the moisture content therein while maintaining the original shape, freshness and taste, the freeze-dried fruit chips are then subjected to microwave-drying under vacuum to thaw the remaining moisture content present in the form of ice crystal and evaporate a predetermined amount of water from the thawed moisture content while uniformly dispersing the thawed moisture content throughout the chips to provide fruit chips having the water-soluble sugar dispersed uniformly therein and the resulting fruit chips are subjected to vacuum-drying to further increase the water-soluble sugar concentration therein.
U.S. Pat. No. 4,096,283 discloses a process for making a dehydrated and compacted food mass selected from the group consisting of vegetables and meats which comprises the steps of partially freeze-drying a food, irradiating the partially freeze-dried food with microwave radiations, compacting the microwave-irradiated food and oven-drying the compacted food. However, the prior art of this U.S. patent is directed to a process for making dehydrated and compacted food such as vegetable or meat which can be rehydrated to the state prior to drying thereof and is quite different from the method for making dry fruit chips ready for eating as snack fruit. Thus, the prior art of this U.S. patent is not applicable to the making of drying fruit chips ready for eating as snack fruit to which the present invention is directed.